1. Field of the Invention
The present invention relates to a modifying agent for a conductive substrate for use in producing a chemically modified electrode.
2. Prior Art
As modifying agents for producing chemically modified electrodes used for efficient reduction of an electron transfer protein, there have been known promotors such as 4,4'-bipyridyl, as described in J. Am. Chem. Soc., 101, p4461 (1979), and, also, mediators such as ferrocene derivatives, as described in J. Am. Chem. Soc., 105, p181 (1983).
FIG. 1(a) shows the structural formula of 4,4'-bipyridyl, and FIG. 1(b) shows the structural formula of a ferrocene derivative.
The operation of the conventional modified electrodes will now be explained. FIG. 2 shows a cyclic voltamogram obtained by immersion of a modified electrode, obtained by modifying gold by the promotor shown in FIG. 1(a), in an aqueous solution of cytochrome c, which is one of electron transfer proteins. In FIG. 2, peaks corresponding to reduction and oxidation of cytochrome c are observed, showing that cytochrome c can be oxidized and reduced by use of the electrode. Similarly, it is possible to oxidize and reduce cytochrome c by use of a modified electrode obtained by modifying platinum by the mediator shown in FIG. 1(b). In FIG. 2, the ordinate indicates current (.mu.A) and the abscissa indicates voltage (mV).
With the conventional promotor-modified electrode constructed as described above, bidirectional electron transfer takes place between cytochrome c and the electrode and, therefore, it is impossible to control the flow of electrons to only one direction, from the electrode to cytochrome c. In the case of the mediator-modified electrode, the method of modification is complicated. Besides, where the difference in oxidation-reduction potential between the modifying agent and the electron transfer protein is small, as in the case of a combination of ferrocene and cytochrome c, bidirectional electron transfer takes place similarly to the above case of the promotor-modified electrode, and it is therefore impossible to control the direction of electron flow.
It is possible to achieve the desired control of the direction of electron flow between a modified electrode and an electron transfer protein such as cytochrome c, by using a flavin derivative having a standard oxidation-reduction potential lower than, or on the negative side of, the standard oxidation-reduction potential of the electron transfer protein to produce the modified electrode. However, it has been difficult to modify a conductive substrate by commercially available riboflavin or lumifravin.